Effects of Re substitution on the structure and superconductivity of Cu 1− xRe xBa 2YCu 2O w

Abstract

Samples of Cu 1− x Re x Ba 2YCu 2O w , with nominal x=0, 0.05, 0.1 and 0.15, and 6.91⩽ w⩽7.17 were prepared and characterized. The aim was to evaluate the possibility of improving the transport and magnetic properties of CuBa 2YCu 2O w (YBCO) by substituting Cu in the reservoir blocks (Cu1) with a high-valent element such as Re. X-ray powder diffraction and energy dispersive spectroscopy (EDS) studies indicated that Re substitutes for Cu1 up to x≈0.1, beyond which it segregates, mostly by forming a cubic phase with a≈8.275 Å. Such an impurity was identified as YBa 2ReO 6 on the basis of the powder x-ray and neutron diffraction data and EDS microanalysis. The crystal structure of Cu 1− x Re x Ba 2YCu 2O w was refined by the Rietveld analysis based on powder neutron diffraction data. It is orthorhombic (Pmmm space group), with a, b and c (≈3.84, 3.88 and 11.70 Å) depending on x and w. The Re cations were found to occupy the (0 0 0) position and to be surrounded by 6 oxygen atoms at the vertices of an octahedron (2O1, 2O4 and 2O5). Such units do not significantly perturb the YBCO structure. No ordering between Cu and Re was observed by electron diffraction. The resistive and magnetic measurements indicated that the optimum oxygen doping of the Re-doped YBCO occurs at w>7. The corresponding Cu average valence is ≈2.27±1%. T c of the optimally doped samples is not very sensitive to the Re content and remains as high as ≈91 K when x=0.1. The residual resistivity extrapolated at T=0 K scales linearly with x, according to the Mathiessen’s rule. SQUID measurements for the x=0.05 and x=0.1 samples indicated that H irr and j c are lower than those of the undoped phase at any temperature.